tcp: usec resolution SYN/ACK RTT

Currently SYN/ACK RTT is measured in jiffies. For LAN the SYN/ACK
RTT is often measured as 0ms or sometimes 1ms, which would affect
RTT estimation and min RTT samping used by some congestion control.

This patch improves SYN/ACK RTT to be usec resolution if platform
supports it. While the timestamping of SYN/ACK is done in request
sock, the RTT measurement is carefully arranged to avoid storing
another u64 timestamp in tcp_sock.

For regular handshake w/o SYNACK retransmission, the RTT is sampled
right after the child socket is created and right before the request
sock is released (tcp_check_req() in tcp_minisocks.c)

For Fast Open the child socket is already created when SYN/ACK was
sent, the RTT is sampled in tcp_rcv_state_process() after processing
the final ACK an right before the request socket is released.

If the SYN/ACK was retransmistted or SYN-cookie was used, we rely
on TCP timestamps to measure the RTT. The sample is taken at the
same place in tcp_rcv_state_process() after the timestamp values
are validated in tcp_validate_incoming(). Note that we do not store
TS echo value in request_sock for SYN-cookies, because the value
is already stored in tp->rx_opt used by tcp_ack_update_rtt().

One side benefit is that the RTT measurement now happens before
initializing congestion control (of the passive side). Therefore
the congestion control can use the SYN/ACK RTT.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 6d8795b066ac..10933d01b982 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -470,7 +470,7 @@ struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req,
                 newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
                 tcp_enable_early_retrans(newtp);
                 newtp->tlp_high_seq = 0;
-                newtp->lsndtime = treq->snt_synack;
+                newtp->lsndtime = treq->snt_synack.stamp_jiffies;
                 newtp->last_oow_ack_time = 0;
                 newtp->total_retrans = req->num_retrans;
 
@@ -760,6 +760,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
         if (!child)
                 goto listen_overflow;
 
+        tcp_synack_rtt_meas(child, req);
         inet_csk_reqsk_queue_drop(sk, req);
         inet_csk_reqsk_queue_add(sk, req, child);
         /* Warning: caller must not call reqsk_put(req);